The present invention relates to means for controlling the flue gas flow pattern created by the flow of the flue gas from a horizontal duct into a vertical flue gas stack, thereby eliminating or significantly reducing any adverse effects resulting from that pattern.
The flow patterns in circular flue gas stacks formed by the flow of the flue gas from a horizontal duct into stack can best be characterized by two counter-rotating vortices issuing from the side walls of the duct, often referred to as a breech, into the stack. These vortices are unstable and interact with each other as the flue gas travels up the stack. The swirling flow in the stack is controlled by one of the two counter-rotating vortices. Flow instabilities result in a momentary change in direction of the swirl as the opposing vortex gains control. This in turn results in pressure pulsations which travel back through the plant equipment. This can have an adverse effect on the operation and structural integrity of the process and equipment.
As one example, gas turbines are often used to provide electric power usually for standby or peaking power. Because the thermal efficiency of gas turbines alone is rather low due to the high exit gas temperature, the gas turbine is most often combined with a heat recovery steam generator and a steam turbine to produce additional electricity. As a combination of a gas turbine cycle and a steam turbine cycle, these systems are referred to as "combined cycles". Gas turbines with heat recovery steam generators are also used to produce process steam in co-generation plants.
In the situation of combined cycles or co-generation, the pressure pulsations previously referred to travel upstream through the heat recovery steam generator and through the inlet duct to the interface with the gas turbine. Although the interaction of the pressure pulsations with the gas turbine are not fully known, it is hypothesized that the pulse is reflected off of the rotating blades of the turbine and then travels back downstream. Measurements have shown that the turbine back pressure can vary as much as 30% depending on the amplitude of the pulse. Of course, such a large variation in back pressure can have a very negative impact on the operating stability of the gas turbine. Furthermore, such pressure swings can have long term risks associated with material fatigue and stress. These same operating and structural problems will also exist to varying degrees with combustion equipment other than combined cycle systems.